There are hardly any old fish left in the ocean – and that’s bad

There are not just fewer fish in the sea: there are disproportionately fewer old fish. A study of fisheries in the seas around the US and Europe has found that their populations of ageing fish have been reduced by an average of 72 per cent.

The researchers looked at 63 fisheries, which had records spanning 24-140 years. To determine the age of fish, they used several techniques including examining otoliths: “stones” in the fish’s ears that grow annual rings rather like a tree.

“The new statistics revealed that the reduction of older fish populations had actually increased by 180 per cent,” says lead author Lewis Barnett of the University of Washington in Seattle.

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In some species, such as Pacific cod, Pacific hake, red snapper and Atlantic cod, the populations of older individuals have fallen by more than 95 per cent.

A 2010 study that looked at the data from 10 fisheries found declines in the aged population of only 25 per cent. A key difference is that Barnett’s team used models to estimate what the populations were like before large-scale fishing began decades or centuries ago, and compared that to their current state – revealing the starker decline in the population of old fish.

Save the old fish

Losing older fish may not sound problematic, because older fish might be past the age of reproduction and even on the verge of death. But in fact, old fish tend to be the largest and produce the most offspring. They are also the most flexible in their behaviour, so they can adapt better to environmental changes.

For instance, the older fish tend to spawn in different times and locations. This means that short-term environmental changes are less likely to severely affect the population.

Having only a few such individuals leaves these fisheries prone to collapse. However, it is possible to preserve these fisheries while maintaining a healthy mix of ages.

One way is to regularly stop fishing in certain areas to let the populations recover. Alternatively, fisheries could use “slot limits”, which regulate both the minimum and maximum sizes that can be harvested, says Barnett.

Marine reserves can also help – although so far there are not many long-term ones. In 2015, Barnett teamed up with Marissa Baskett of the University of California, Davis, to review how such reserves affected fish demographics. They found that fish in reserves survived to greater ages and that the populations became more genetically diverse – both of which protect the species.